1. Field of the Invention
The present invention relates to a frame for motorcycles includes a first frame section to support a front fork, and a second frame section to support rear arms, and more particularly, to a construction, in which the first frame section and the second frame section are connected to each other.
2. Description of Related Art
Since a motorcycle for moto-cross races, for example, travels on rough terrain having extreme irregularities, a large impact load and a load of fatigue failure are applied on a frame through a front fork and rear arms. Therefore, in order to improve a motorcycle in kinematical performance, it is necessary to enhance the stiffness and strength of the frame while achieving lightening of the frame.
As for a frame which meets with such demands, there are conventionally known frames, in which a pivot frame to support the rear arms is constructed by a combination of casting and forging. Such pivot frame includes a pair of pivot plates to pivotally support the rear arms and an upper cross member connected to a main pipe. The pivot plates include a forging and have a cross sectional shape to be opened inward in a direction along a vehicle breadth. The upper cross member includes a casting and has a hollow closed, cross sectional shape.
The pivot plates are opposed to each other with the rear arms therebetween and include a fitted recess at an upper end thereof. The upper cross member is positioned between the upper ends of the pivot plates. The upper cross member has a pair of arms, which are bifurcated right and left to project, and fitting projections are formed at tip ends of the respective arms. The fitted recesses of the pivot plates are fitted laterally onto the fitting projections of the upper cross member. The upper end of the pivot frame is welded at an edge thereof to the arms of the upper cross member whereby the pivot frame and the upper cross member make a unitary structure.
The pivot frame is welded to the upper cross member in a state of covering the fitting projections of the upper cross member in a lateral direction. Therefore, the upper cross member is connected to the pivot frame while maintaining a hollow closed, cross sectional shape.
Since the pivot frame composed of a forging has a cross sectional shape to be opened inward in the direction along the vehicle breadth, however, it is not possible to avoid the situation, in which the frame is extremely varied in cross sectional shape at the connection of the upper cross member and the pivot frame. In other words, a cross sectional shape of the frame changes extremely from a closed cross section to an opened cross section at the connection of the upper cross member and the pivot frame, so that at the connection, the frame is greatly changed in strength and stiffness.
As a result, when a high load generated at the time of, for example, landing in jump acts on the frame, there occurs a phenomenon that the entire frame twists about the connection of the upper cross member and the pivot frame, thus causing a disadvantage having an adverse influence on the kinematical performance of the motorcycle.
The invention has been thought of based on the above situation, and has its advantage to provide a frame for motorcycles, capable of preventing an extreme change, in cross sectional shape, of a connection of a first frame section and a second frame section.